The air at atmospheric
pressure is used as an arc extinguishing medium in Air-Break
Circuit-Breakers. These circuit breaker employ the high resistance
interruption principle. the arc is rapidly lengthened by means of the
arc runners and arc chutes and the resistance of the arc is increased
by cooling, lengthening and splitting the arc. The arc resistance
increases to such an extent that the voltage drop across the arc becomes
more than the supply voltage and the arc gets extinguished. Magnetic
field is utilized for lengthening the arc in high voltage air-break
circuit-breakers.
Air-break circuit breakers are used in d..c. circuit and a.c. circuits up to 12KV.
The air-break circuit breakers are generally indoor type and installed
on vertical panels or indoor draw-out type switchgear.
A.C. air circuit breaker are widely used in indoor medium voltage and
low voltage switchgear. Typical reference values of ratings of air-break
circuit breaker are :
460 V , 400-3500 A, 40-75 KA
3.3 KV , 400-3500 A, 13.1-31.5 KA
6.6 KV , 400-2400 A, 13.1-20 KA
5.1.1 Construction of Air-break Circuit-breaker
In the air-break circuit-breaker the contact separation and arc
extinction takes place in air at atmospheric pressure. As the contacts
are opened, arc is drawn between them. The arc core is a conducting
'path' of plasma. The surrounding medium contains ionized air. By
cooling the arc, the diameter of arc core is reduced. The arc is
extinguished by lengthening the arc, cooling the arc and splitting the
arc. The arc resistance is increased to such an extent that the system
voltage cannot maintain the arc and the arc get extinguished.
Figure 4 illustrates the normal arrangement of an air break circuit breaker.
This type of breaker is used for medium and low voltages.
There are tow sets of contacts : Main contacts (1) and Arcing contacts
(2). Main contacts conduct the current in closed position of the
breaker. They have low contact resistance and are silver plated. The
arcing contacts (2) are hard, heat resistance and are usually of copper
alloy. While opening the contacts, the main contacts dislodge first. The
current is shifted to the arcing contacts. The arcing contacts dislodge
later and arc is drawn between them (3). This arc is forced upwards by
the electromagnetic forces and thermal action. The arc ends travel along
the arc Runners (Arcing horns). The arc moves upwards and is split by
arc splitter plates (arc chutes) (5) as shown by the arrow (4). The arc
extinguished by lengthening, cooling, splitting etc. In some breakers
the arc is drawn in the direction of the splitter by magnetic field.
Furthermore,
air-break circuit-breaker have been developed with current limiting
feature, magnetic blow-up of arc, etc. Air break a.c. circuit breakers
are widely used for industrial switchgear, auxiliary switchgear in
generation station. Air break principle employing lengthening of arc,
arc runners, magnetic blow-ups are used for d.c. circuit breaker up to
15 KV.
